Atomic-Resolution Simulations Show Two Sequential Fusion Peptide Mechanisms in Influenza Membrane Fusion

Abstract

Influenza viral membrane fusion is mediated by the hemagglutinin protein, and membrane-inserted fusion peptides are critical to the success of this process. However, the field lacks a mechanistic understanding of how fusion peptides act to cause fusion that can fully explain experimental phenotypes of fusion peptide mutants. We have performed the first atomic-resolution simulations of influenza membrane fusion, which demonstrate lipid tail protrusion and membrane bending as two sequential activities of the fusion peptides that are critical to fusion. Each of these is supported by previous experimental data and mechanistic hypotheses, but our simulations now place these, in sequence, as co-existing mechanistic requirements for fusion. A striking new finding is that the membrane-bending activity of fusion peptides is dependent on their N-terminal protonation state, which can in turn control the helical kink or hairpin that modulates fusion activity. These simulations thus provide a unified model for influenza membrane fusion capable of explaining how N-terminal fusion peptide mutations exert their effects. Furthermore, our model by which fusion peptides promote lipid tail protrusion and then membrane bending as sequential requirements may be generalized to explain the fusion behavior of other enveloped viruses.